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Using new large-area maps of the cold neutral medium (CNM) fraction,f_CNM, we investigate the relationship between the CNM, the abundance of polycyclic aromatic hydrocarbons (PAHs), and the anomalous microwave emission (AME). We first present ourf_CNMmap based on full-sky HI4PI data, using a convolutional neural network to convert the spectroscopic Hidata tof_CNM. We demonstrate thatf_CNMis strongly correlated with the fraction of dust in PAHs as estimated from mid- and far-infrared dust emission. In contrast, we find no correlation betweenf_CNMand the amount of AME per dust emission and no to weakly negative correlation betweenf_CNMand the AME peak frequency. These results suggest PAHs preferentially reside in cold, relatively dense gas, perhaps owing to enhanced destruction in more diffuse media. The lack of positive correlation betweenf_CNMand AME peak frequency is in tension with expectations from theoretical models positing different spectral energy distributions of AME in the cold versus warm neutral medium. We suggest that different PAH abundances and emission physics in different interstellar environments may explain the weaker-than-expected correlation between 12μm PAH emission and AME even if PAHs are the AME carriers.

We compare observations of Hifrom the Very Large Array (VLA) and the Arecibo Observatory and observations of HCO⁺from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) in the diffuse (A_V≲ 1) interstellar medium (ISM) to predictions from a photodissociation region (PDR) chemical model and multiphase ISM simulations. Using a coarse grid of PDR models, we estimate the density, FUV radiation field, and cosmic-ray ionization rate (CRIR) for each structure identified in HCO⁺and Hiabsorption. These structures fall into two categories. Structures withT_s< 40 K, mostly withN(HCO⁺) ≲ 10¹²cm⁻², are consistent with modest density, FUV radiation field, and CRIR models, typical of the diffuse molecular ISM. Structures with spin temperatureT_s> 40 K, mostly withN(HCO⁺) ≳ 10¹²cm⁻², are consistent with high density, FUV radiation field, and CRIR models, characteristic of environments close to massive star formation. The latter are also found in directions with a significant fraction of thermally unstable Hi. In at least one case, we rule out the PDR model parameters, suggesting that alternative mechanisms (e.g., nonequilibrium processes like turbulent dissipation and/or shocks) are required to explain the observed HCO⁺in this direction. Similarly, while our observations and simulations of the turbulent, multiphase ISM agree that HCO⁺formation occurs along sight lines withN(H I) ≳ 10²¹cm⁻², the simulated data fail to explain HCO⁺column densities ≳ few × 10¹²cm⁻². Because a majority of our sight lines with HCO⁺had such high column densities, this likely indicates that nonequilibrium chemistry is important for these lines of sight.

We have complemented existing observations of Hiabsorption with new observations of HCO⁺, C₂H, HCN, and HNC absorption from the Atacama Large Millimeter/submillimeter Array and the Northern Extended Millimeter Array in the directions of 20 background radio continuum sources with 4° ≤ ∣b∣ ≤ 81° to constrain the atomic gas conditions that are suitable for the formation of diffuse molecular gas. We find that these molecular species form along sightlines whereA_V≳ 0.25, consistent with the threshold for the Hi-to-H₂transition at solar metallicity. Moreover, we find that molecular gas is associated only with structures that have an Hioptical depth >0.1, a spin temperature <80 K, and a turbulent Mach number ≳ 2. We also identify a broad, faint component to the HCO⁺absorption in a majority of sightlines. Compared to the velocities where strong, narrow HCO⁺absorption is observed, the Hiat these velocities has a lower cold neutral medium fraction and negligible CO emission. The relative column densities and linewidths of the different molecular species observed here are similar to those observed in previous experiments over a range of Galactic latitudes, suggesting that gas in the solar neighborhood and gas in the Galactic plane are chemically similar. For a select sample of previously observed sightlines, we show that the absorption line profiles of HCO⁺, HCN, HNC, and C₂H are stable over periods of ∼3 yr and ∼25 yr, likely indicating that molecular gas structures in these directions are at least ≳100 au in size.